1. Technical Field
This disclosure generally relates to the field of analog switching.
2. Description of the Related Art
Switches play an important role in the design of circuits. Generally a switch is used to pass a signal from an input terminal of the switch to the output terminal of the switch. This use of a switch generally has two states. The first state corresponds to when the switch is passing a signal, and the second state corresponds to when the switch is not passing a signal.
FIG. 1A shows an example of a switch configuration. In this example, the switch configuration is that of a transmission gate. The transmission gate of FIG. 1A comprises complementary transistors coupled in parallel to each other. In this example, switch 10 is an n-channel metal-oxide semiconductor (“NMOS”) transistor. Switch 14 is a p-channel metal-oxide semiconductor (“PMOS”) transistor. As shown in FIG. 1A, the sources of the transistors are coupled together as an input terminal, and the drains of the transistors are coupled together as an output terminal. The control terminals, or gates, of the transistors are coupled to respective control signals. A load resistor 18 represents a resistance that may coupled to a typical output of a switch configuration such as that found in FIG. 1A.
The transmission gate of FIG. 1A is a beneficial configuration of a switch because the combination of the switch 10 and switch 14 allows for a full swing of the input signal to be represented on the output. The switch 10 fully passes voltage signals ranging down to or near a low voltage rail, and switch 14 fully passes voltage signals ranging up to or near a high voltage rail.
FIG. 1B represents a small signal model of the circuit in FIG. 1A. A voltage source 20 is coupled to the input of the transmission gate and is applying a voltage through a voltage source resistance 22. Channel resistance 24 represents the channel resistance of switch 10 in parallel with switch 14. The output capacitance 26 represents the output capacitance of switch 10 in parallel with the output capacitance of switch 14, and load resistance 18 of FIG. 1B represents load resistor 18 shown in FIG. 1A.
Looking at both FIG. 1A and FIG. 1B, consider the state where control signal NG_CTL is high and its complimentary control signal PG_CTL is low, so that switch 10 and switch 14 are fully on. With channel resistance 24 of FIG. B representing an on-resistance for the switches, the bandwidth of the switch configuration can be estimated. The voltage source resistance 22 is approximately equal to the load resistance 18, each having a value R. With the channel resistance 24 having a value of R0 and output capacitance 26 having a value of C, a time constant τ can be calculated as follows: τ=((R+R0)∥R)C. Neglecting the comparatively channel resistance 24, R0, results in an equivalent resistance of approximately
      R    2    .Thus the approximate time constant of the single time constant circuit is
  τ  =            RC      2        .  Because bandwidth can be estimated as being approximately
      1    τ    ,the bandwidth of this circuit is approximately
      2    RC    .It should be noted that the bandwidth of this single time constant low-pass filter circuit is inversely proportional to the resistance and the capacitance of the circuit. The capacitor C affects the circuit output when current begins flowing through the capacitor to ground. Thus, the ability to mitigate the affect of the capacitor by reducing current flowing through it will result in increasing the bandwidth of the circuit.
Conventionally, to increase the bandwidth of an analog switch, a series resistor is inserted between the gate of the transistor and some gate control circuit. FIG. 2A shows a single-transistor configuration of a switch and includes switch 10, control terminal resistor 28, and load resistor 18. In order to achieve high bandwidth, either an NMOS or PMOS transistor is used as a switch, instead of a complimentary pair. The selection of the device will depend upon the input signal range to be used in the design. However, NMOS carriers are smaller and are more mobile than their PMOS counterparts, so NMOS transistors are considered more frequently than PMOS transistors for implementation in high frequency switches.
FIG. 2B represents a small signal model of the circuit in FIG. 2A. FIG. 2B includes voltage source 20, a voltage source resistance 22, load resistor 18, and a switch model inclusive of control terminal resistor 28. The switch model comprises channel resistance 30, oxide capacitance 32, control terminal resistor 28, and capacitance 36. The effect of the control terminal resistor 28 can be seen when an AC signal is forced into the input of the switch. Current passing through a capacitor increases with increase in frequency as
  (      1          j      ⁢                          ⁢      ω      ⁢                          ⁢      C        )and effectively acts as a short in high frequencies. However, rather than the capacitance 32 shorting to AC ground, the current is directed through control terminal resistor 28, thereby producing a voltage drop across the resistor. This voltage drop effectively maintains a voltage on control terminal 12 of switch 10, allowing the transistor to continue operating until other parasitic effects attenuate the signal. FIG. 2C charts the bandwidth of a design of switch 10 against various values of control terminal resistor 28. As is shown, a conventional configuration for increasing bandwidth in an analog switch is performed by placing a resistor in series with the control terminal of the switch.
Other configurations used to increase the bandwidth of a switch include the use of additional transistors to control the bulk of the device. FIGS. 3A, 3B, and 3C represent different configurations of bulk control circuits used to increase the bandwidth of a switch. In these configurations, the main purpose is to reduce the parasitic capacitance seen from the source to the body and seen from the drain to the body of switch 10.
FIG. 3A includes switch 10, control terminal resistor 28 and bulk transistors 38, 40, and 42. While control signal NG_CTL holds switch 10 on, bulk transistors 38 and 40 provide a low resistance connection to between input terminal 15 to bulk and between the output terminal to bulk, respectively. This configuration of a switch can be used in parallel with other similar versions of the switch. When switch 10 is off bulk transistor 42 provides a low resistance connection between the body of switch 10 and ground. Said connection allows other switch configurations coupled in parallel to use switch 10 as capacitive loading.
FIG. 3B includes switch 10, control terminal resistor 28 and bulk transistors 42 and 44. In this configuration, the resistance from bulk to ground can be set at different values for different transistor states. For example, by setting the on-resistances of bulk transistors 42 and 44 to be different from one another, switch 10 may be configured with different bulk to ground resistances. Switch 10 may configured with one bulk to ground resistance through bulk transistor 44 when switch 10 is on, and it may be configured with another bulk to ground resistance through bulk transistor 42 when switch 10 is off. These defined channel resistances reduce the parasitic capacitances seen from the input and output terminals to bulk.
FIG. 3C is a variation of the switch configuration of FIG. 3B. FIG. 3C shows bulk transistor 46 tied to rail voltage VCC. This bulk circuit configuration allows current flowing through the input and output terminal capacitances of switch 10 to flow through the channel resistance of bulk transistor 46 before passing to ground. While switch 10 is held off, i.e., control signal NG_CTL is low, the bulk resistance is decreased by holding bulk transistor 42 on in parallel with bulk transistor 46, thereby increasing the capacitive loading of devices sharing a common output with switch 10.
In summary, there are several configurations that have attempted in the past or conventionally to increase the bandwidth of an analog switch. However, each of these configurations have particular shortcomings which they do not overcome.